The present invention relates to an electrical contact element for electrically connecting an electric device and providing current transmission in an electric circuit. The electrical contact element comprises a metallic body, where at least one contact surface present on the body is completely or partially coated with a friction-reducing layer.
Electrical contact elements, such as, for example, connection and terminal devices of plug-in type, sliding contacts or stationary contacts, which are adapted for electrically connecting an electric device and providing current transmission in switchgear for low voltage, medium voltage or high voltage, in a control system or in some form of electric circuit, are preferably manufactured of copper or aluminium. A contact element may, for example, be of lamellar type, of pin type, a spiral contact element, or a device for connection to various types of internal or external busbars.
To improve the electrical and thermal properties of a contact transition while at the same time protecting a contact surface, present on the contact element, against wear and corrosion as well as ensuring the function of the contact element for a longer period of time, it is known to coat the contact element with, for example, silver or tin. However, a silver- or tin-coated contact surface, which slides against another silver- or tin-coated contact surface, exhibits a great tendency to become welded together. To avoid this problem, the silver- or tin-coated contact surfaces are therefore usually lubricated with a lubricant. For silver- or tin-coated contact surfaces, a lubricant with an oil or a fat as base is usually used. Solid lubricants, for example graphite or various types of plastics, may also be used. However, solid lubricants are poor electric conductors and are often worn off when the contact surfaces slide against each other.
Also for contact elements with an uncoated contact surface, it is desirable with reduced friction in the contact surface.
U.S. Pat. No. 5,316,507 provides an example of a contact layer with lubricating properties. A solid lubricant, graphite of a certain particle size, mixed with a powder of an electrically conductive material, for example gold, is pressed into a body which is sintered. During the sintering, the gold grains fuse and the graphite remains in cavities of the gold. The sintered body is rolled into a strip in a pluralilty of rolling steps with intermediate heat-treatment operations, the strip being used as a conductive and lubricating contact layer for contact elements. A disadvantage with the above-mentioned conductive and lubricating contact layer is that it requires a complicated and expensive manufacturing process.
One problem when using a fat- or oil-based lubricant is that it is difficult to apply an even layer of lubricant to the contact surface. Thick films of the lubricant have an adverse influence on the electrical properties and thin films of the lubricant are often worn away by mechanical influence. Another problem when using a lubricant is that it is volatile and thus contaminate other components. Still another problem is that the lubricant is sticky, which implies that it adheres to components which should not be lubricated and that it easily absorbs contaminants, for example particles of dust, which may give an increased contact resistance. The contaminants in the lubricant may also lead to the lubricant oxidizing more easily and thus becoming less durable.
One object of the invention is to achieve an electrical contact element with a friction-reducing layer without the above-mentioned disadvantages when using a fat- or oil-based lubricant for friction-reducing purposes. The friction-reducing layer shall have a low friction at the contact surfaces, a good resistance to wear and a high corrosion resistance. The friction-reducing layer shall also be simple and inexpensive to manufacture.
An electrical contact element adapted to electrically connect an electric device and provide current transmission in switchgear for low voltage, medium voltage or high voltage, in a control system or in some form of electric circuit, comprises a metallic body, preferably of copper, aluminium or an alloy based on any of these two metals. To improve the electrical and thermal properties of the contact transition while at the same time protecting a contact surface, present on the contact element, against wear and corrosion and ensuring the function of the contact element for a longer period of time, it is common practice to coat at least one of the contact surfaces present on the body at least partially with a layer of, for example, silver or tin. According to the invention, the contact element, possibly with a layer of silver or tin, is coated with a friction-reducing layer comprising a metal compound, preferably a metal salt, as, for example, a metal halogenide or a metal sulphide. Examples of metal halogenides are, for example, silver halogenide, tin halogenide or copper halogenide. Examples of silver halogenides are silver iodide, silver chloride or silver bromide. Corresponding halogenides also exist for copper and tin. The friction-reducing layer has a thickness which is in the interval of from 0.001 xcexcm to 1000 xcexcm and preferably is smaller than 5 xcexcm.
A contact element according to the invention exhibits a reduced friction for the contact surfaces, an improved resistance to wear, and an increased corrosion resistance of the contact surfaces compared with a non-coated silver layer or tin layer. It has proved, for example, that the friction at the contact surfaces is considerably lower for the coating of the metal salt, such as the metal halogenide or the metal sulphide, compared with the friction for a layer of, for example, tin or silver on the contact surface. In case of low friction at the contact surfaces, the operation of the contact element is facilitated and enables the use of higher contact forces. A low friction at the contact surfaces also results in increased resistance to wear, which leads to improved electrical and thermal properties and increases the expected service life.
The coating of the metal salt, such as the metal halogenide or the metal sulphide, is carried out, for example, electrolytically directly on the metal body of, for example, copper or aluminium, or after the metallic body has become silver-plated or tin-coated. The coating of the metal salt is also obtained by dipping the contact element into a solution comprising at least one of the following ions: chloride, bromide, iodide or sulphide. The halogen or the sulphide reacts with the metal at the contact surface and forms the layer of the metal salt.
Another possible coating method is evaporation. During evaporation, the contact element is placed in a closed chamber and a gas comprising at least one of the halogens or the sulphide is released into the chamber. The halogen or the sulphide reacts with the surface of the contact element and forms a solid compound of metal halogenide or metal sulphide on at least part of the contact surface.
A further example of a coating process which may be used is CVD (Chemical Vapour Deposition). In the CVD process, the material with which the whole of, or parts of, the contact surface is to be coated is evaporated. By a chemical reaction on or in the vicinity of that surface which is to be coated, a solid layer is formed on the surface. By means of the CVD process, the layer is given a uniform thickness with a low porosity.
One advantage of coating the contact surface with a layer of a chemical compound as metal halogenide or metal sulphide, compared with using a fat- or oil-based lubricant, as according to the prior art, is that the layer with the metal halogenide or the metal sulphide has a longer durability than the layer with the fat- or oil-based lubricant. Another advantage is that the contact element is lighter to handle after the coating with the metal halogenide or the metal sulphide than after the coating with the fat- or oil-based lubricant. The fat- or oil-based lubricant is volatile and easily creeps to surfaces which should be free from lubricant. Additional advantages of the layer of the metal halogenide or the metal sulphide are that a more uniform layer may be obtained and that the layer has better adhesion than the layer of the fat- or oil-based lubricant.